The best way to get a child to say the /r/ sound is whatever way works. Having said this, there are strategies that promote quicker acquisition of /r/. How to Teach a Child to Say the “R” Sound in 15 Easy Lessons, focuses on lifting the tongue tip up and back. A second popular approach is to retract the back of the tongue so that it humps while the front of the tongue slides back along the floor of the mouth. However, neither of those will work if one important element is missing—tongue tension.
Tongue tension is a difficult concept for a child under the age of 10 to grasp. Using other terms, such as strong or tight, can be helpful but may miss the mark. So, how does one convey the concept of tension? I suggest using proprioceptive feedback. This can be easily accomplished by asking the child to show you how she makes a strong muscle with her arm. From there, you can explain that the tongue is just one big muscle and it can be strong like the arm muscle. This is where the choice of approaches comes in. I like to have the child raise the tongue tip up and back because it resembles what was done with the arm (forearm coming up and back). Once the child’s tongue is in the correct position, I tell the child to make her tongue really strong. Now we should have the tension needed for /r/!